Materials with simultaneous high electrical conductivity and optical transmittance are essential for numerous optoelectronic devices, such as flat panel displays (TVs and computer monitors), touch screen displays (smart phones and tablets), thin film solar cells, light-emitting diodes and electromagnetic shielding. The predominant material used for such a transparent conductive electrode (TCE) is tin-doped indium oxide (ITO). ITO has optical transmittance greater than 80% in the visible spectrum, and so-called sheet resistance of about 10 ohm/square. Any other material or process that wishes to compete for TCE market share should reach or beat those metrics. There are two major reasons researchers are working to develop a replacement for ITO as a TCE: (1) ITO contains indium, a rare earth metal that is relatively rare and therefore expensive (about $1 per gram), as well as relatively brittle, limiting its use in flexible applications; and (2) fabrication of ITO and related TCE's requires vacuum-based deposition processes, such as physical vapor deposition (e.g. sputter deposition or thermal evaporation) or chemical vapor deposition. The cost of equipment for large scale implementation of these processes adds to the cost of the TCE, and thus to that of the end product.
There is ample evidence that an all solution-processed replacement for ITO is desired by many markets. The transparent conductive electrode, or transparent conductive film (TCF) market has been forecast to continue to reach over $5 billion by 2020.
Previously, one related process was described in which a film is applied to a substrate (typically a transparent substrate such as glass), the film is cracked by some process, and a conducting material (typically a metal) is deposited onto the cracked film. Some of that conducting material deposits into the voids created by the cracks, thus reaching the substrate. The next step, removing the initial film (which typically has the deposited conducting material on it), is intended to leave only the conducting material that had filled the cracks behind and attached to the substrate, thus forming a type of “metal mesh”. This mesh can be both highly optical transmitting and highly electrically conducting, thus functioning as a TCE.
The art currently lacks a process for fabrication of a TCE that contains no indium and for which the deposition processes are all solution-based, addressing both major issues described above for the need to develop a replacement for ITO as a TCE.